A common method of imparting a wood smoke flavour to foods is by contacting the food directly with the smoke produced upon combustion of wood or sawdust. The food, thus exposed to the smoke, gradually takes on the desired smokey flavour and character, and also a smoked appearance.
The smoke produced from wood fires in smoke generators cannot readily be controlled and the flavour imparted depends on a number of variable factors, including intensity and type of wood smoke, the area, time and temperature of contact, and the extent of deposition of wood tars on the food surface.
For these reasons, there have been developed aqueous smoke flavour solutions, or "liquid smoke", for use in achieving the controlled imparting of flavour to the food product being treated by contact therewith.
In U.S. Pat. No. 3,106,473, there is described a process for producing an aqueous wood smoke flavoured solution in which wood smoke produced on burning sawdust is subjected to repetitive countercurrent extraction with water.
U.S. Pat. No. 3,873,741 describes a controlled carbonizing combustion of sawdust in the presence of a regulated air supply, and an aqueous extraction system, so as to obtain a liquid having improved flavour and an enhanced ability to colour food products treated therewith. The product is said to be readily dilutable, produced in high yield and completely free from carcinogenic materials.
U.S. Pat. No. 4,298,435 indicates that problems were associated with the batch processes described in U.S. Pat. No. 3,873,741, including the variability of the quality of the liquid smoke product obtained and a serious fire and explosion hazard associated with the large amounts of air used therein. In the process described in U.S. Pat. No. 4,298,435, dry hardwood sawdust having a moisture content below 5% is fed into one end of a rotary calciner wherein it is calcined at a temperature below 925.degree. F. (about 500.degree. C.) in the absence of added air.
Smoke produced by the combustion passes into a packed tower through which an aqueous liquid containing condensed smoke is recycled until the desired smoke concentration is reached, along the lines described in U.S. Pat. No. 3,106,473. Tar is allowed to settle out to eliminate carcinogens.
In all the prior art liquid smoke producing processes of which the applicants are aware, char is produced along with the liquid smoke, possibly as a result of decomposition of volatiles in the heating chamber. The production of the char as well as non-condensible combustion products significantly decreases the theoretical yield of liquid smoke product based on sawdust. Generally, the prior art processes produced yields of liquid smoke based on sawdust of only about 45 to 50%.